DESCRIPTION: (Applicant's Abstract) The broad, long-term objective of this application is to design better treatment for large granular lymphocyte (LGL) leukemia. LGL leukemia is a clonal lymphoproliferative disease associated with neutropenia and autoimmune features. Altered expression of Fas receptor (Fas) and Fas ligand is associated with lymphoproliferative and autoimmune diseases in preclinical models. Preliminary data suggest that dysregulation of the Fas-Fas ligand apoptotic pathway underlies the pathogenesis of LGL leukemia. Leukemic LGL, like T cells from Fas ligand transgenic mice, constitutively express high levels of Fas, yet are resistant to Fas-dependent apoptosis. Sera from LGL leukemia patients contain high levels of Fas ligand and cause apoptosis of normal neutrophils in a Fas-dependent fashion. These data suggest that LGL leukemia can serve as a model of Fas-Fas ligand dysregulation causing human disease. The goal of this application is to investigate this model by pursuing correlative laboratory studies associated with a therapeutic trial. Patients with neutropenia or anemia will initially receive oral methotrexate. Patients failing to respond to methotrexate will then receive oral cyclophosphamide. Each specific aim will test a postulated mechanism of treatment efficacy, as follows: Specific Aim 1: To determine if treatment results in apoptosis of leukemic LGL; Specific Aim 2: To determine if treatment results in decreased secretion of Fas ligand; Specific Aim 3: To determine if treatment leads to elimination of the leukemic clone. In Specific Aim 1, it will be determined whether leukemia LGL become sensitive to apoptosis with in vitro treatment and whether Fas resistance can be explained by defects in Fas signaling. Determination of Fas expression, levels of soluble Fas, and assessment of apoptosis will be monitored on treatment. In Specific Aim 2, several complementary assays to detect Fas ligand will be assessed to determine if response to therapy is related to decreased levels of Fas ligand. In Specific Aim 3, the joining sequences of rearranged T cell receptor genes utilized by leukemic LGL will be determined to see if there is molecular evidence for antigen-driven proliferation. Efficacy of treatment will be assessed using these leukemia- specific sequences in a PCR assay aimed at detecting minimal residual disease. Results of these studies may suggest that novel strategies targeting Fas ligand would be efficacious for treatment of malignancies and autoimmune diseases.